The invention relates to color projection systems, and more particularly to projection systems incorporating a single electro-optical light modulator.
Color projection display systems exist in which a white light source is separated into red, blue, and green sub-beams for separate modulation by corresponding color components of an incoming display signal, and then the modulated subbeams are recombined into a full color display for projection onto a viewing screen. Modulation of the subbeams is commonly carried out using three separate electro-optical light modulators such as liquid crystal display (LCD) panels, one for each of the three subbeams.
However, in one type of color projection system the three subbeams are all modulated by a single LCD panel. This is accomplished by shaping the subbeams into band-shaped cross-sections, and scrolling the bands sequentially across the LCD panel (also referred to as a light valve), while synchronously addressing those portions of the panel that are illuminated by the bands with the corresponding display signal information. The simultaneous use of a substantial portion of the available red, blue and green light through a single light valve panel provides optical efficiencies comparable to that of three-panel systems employing the same types of light-valve panels. Using only a single panel eliminates the need to mechanically converge different color images, formed on different panels, and reduces system cost.
A compact apparatus for generating a scrolling color stripe pattern is taught by Gleckman in U.S. Pat. No. 6,266,105 for example. Gleckman""s apparatus employs a drum covered with dichroic elements that selectively reflect red, green and blue color bands. The reflected light is separated from the incoming light by means of a polarizing beam-splitter and quarter wave plate. Unfortunately however, neither the dichroics nor the optics can be produced at low cost.
Our new invention does not use expensive elements and does not rely on polarized light. It is based on low-cost technology and can also be used with light valves that don""t use polarized light, e.g. tilting mirror array or DMD. Generally, multistripe scrolling according to the invention includes using holographic elements to isolate light beams of three colors, e.g., red, green, and blue, from white light, and causing them to scroll sequentially across a light valve.
In one aspect of the invention, a multi-stripe scrolling apparatus comprises a white light source; a lens system including a total internal reflection beam splitter having an internal surface exhibiting a critical angle of total internal reflection; a movable array of holographic elements including a plurality of first holographic elements for producing a first color, a plurality of second holographic elements for producing a second color substantially different from the first color, and a plurality of third holographic elements for producing a third color substantially different from the first and second colors; and a light valve. Each of the first, second, and third holographic elements is configured such that white light arriving at a respective one of the first, second, and third holographic elements from a respective arrival direction produces a beam of substantially monocolor light of a respective one of the first, second, and third colors, that leaves the respective one of the first, second, and third holographic elements in a respective departure direction that differs by a predetermined angle from the respective arrival direction. The predetermined angle is the same for the first holographic elements as for the second and third holographic elements. The white light source, lens system including beam splitter, movable array, and light valve are physically disposed relative to one another such that light from the white light source enters the beam splitter and impinges at a angle of incidence to the internal surface that is greater than the critical angle, so that the light from the white light source is reflected from the internal surface and is directed to the respective ones of the first, second, and third holographic elements, and such that the a beams of substantially monocolor light leaving respective first, second, and third holographic elements reach the beam splitter and impinge at a second angle of incidence to the internal surface that is less than the critical angle, so that the beams of first, second, and third color light pass through the internal surface to form alternating bands of light of the first, second, and third colors that scroll across the surface of the light valve when the movable array is moving.
In another aspect of the invention, a multi-stripe scrolling apparatus comprises a white light source; a total internal reflection-beam splitter disposed to internally reflect light from the white light source; a movable array of holographic elements including a plurality of first color-emitting holographic elements, second color-emitting holographic elements, and third color-emitting holographic elements alternatingly arranged and disposed to receive the light internally reflected by the beam splitter, and to emit first, second, and third color light beams that are not internally reflected but pass through the beam splitter, the first, second, and third colors being substantially different colors from one another; and a light valve disposed to receive the first, second, and third color light beams that have passed through the beam splitter.
In yet another aspect of the invention, a multi-stripe scrolling apparatus comprises a light valve; means for generating light; holographic element means for receiving the light and producing therefrom first, second, and third color light beams of respectively different colors; and means for internally reflecting the light off an internal surface, for directing the reflected light to the holographic element means, for transmitting the first, second, and third color light beams through the internal surface, and for directing the first, second, and third color light beams onto the light valve to form alternating bands of light of the first, second, and third colors across the surface of the light valve.
In still another aspect of the invention, a method of causing alternating bands of first, second, and third color light to scroll-across the surface of a light valve, comprises reflecting a collimated beam of light off an internal surface of a total internal reflection beam splitter; directing the reflected light onto an array of holographic elements including at least a first, a second, and a third holographic element; emitting a first color light beam from the first holographic element; emitting a second color light beam from the second holographic element, the second color being substantially different from the first color; emitting a third color light beam from the third holographic element, the third color being substantially different from both of the first and second colors; transmitting the first, second, and third color light beams through the internal surface of the beam splitter; directing the first, second, and third color light beams transmitted through the internal surface to form alternating bands of light of the first color, the second color, and third color, respectively, on the light valve; and moving the array of holographic elements so that the alternating bands of first, second, and third color light scroll across the surface of the light valve.